In recent years, there has been an increased demand forhigh temperature oxygen sensors, mainly for the monitoring and control of combustion processes, such as the combustion of hydrocarbons in an internal combustion engine. One device of this type widely used for automotive engine control is an electrochemical oxygen concentration cell, usually made of zirconia (ZrO.sub.2). In the most common configuration of this device, the ZrO.sub.2 electrolyte is in the form of a thimble with one side exposed to the combustion environment and the other exposed to air as a reference atmosphere. This device provides an EMF output which is proportional to the logarithm of the oxygen partial pressure in the combustion, environment. Such a cell is generally termed an oxygen concentration cell.
Despite its low sensitivity, this device is widely used on automobile engines to control and maintain the air-to-fuel mixture in the engine cylinders at the stoichiometric value. A stoichiometric mixture contains just enough oxygen to burn the fuel completely to carbon dioxide and water. The satisfactory operation of this device arises from the fact that the oxygen partial pressure in the product of combustion (exhaust gas) changes by many orders of magnitude as the air-to-fuel mixture is varied through the stoichiometric value.
On the other hand, for the purpose of reducing fuel consumption, it is generally desirable to operate internal combustion engines with "lean" air-to-fuel mixtures, which contain excess air. For these lean mixtures, the oxygen partial pressure after combustion exhibits only a small and gradual change with change in the air-to-fuel mixture. These small changes cannot be easily measured with the above-mentioned oxygen concentration type device. One approach for obtaining high sensitivity devices for use in lean air-to-fuel operation is to employ a so-called oxygen-pumping scheme. Such oxygen-pumping is based on the fact that if a current is passed through an oxygen-conducting electrolyte, (e.g. zirconia), oxygen is transferred (pumped) from one side of the electrolyte to the other. Several oxygen sensors based on this principle have been described in the prior art. Examples are those described in U.S. Pat. Nos. 3,923,624 to Beckman et al; 3,654,112 to Beckman et al; 3,907,657 to Heijne et al; and 3,698,384 to Jayes.
Recently, a series of U.S. patents to Hetrick and Hetrick et al. (U.S. Pat. Nos. 4,272,320; 4,272,330; and 4,272,331) describe an oxygen-pumping device that has improved characteristics over previously described devices, e.g., higher speed of response, lower sensitivity to temperature variations and independence from ambient total pressure changes. These features make this device particularly useful for automotive engine use. This device has two pieces of dense zirconia sealed together to form a cavity that communicates with the outside environment through one or more apertures. Electrodes are deposited on the inside and outside walls of each of the two sections of the device, thus forming an oxygen pumping cell and a sensing cell. Still another oxygen pumping sensor is disclosed in U.S. Pat. No. 4,487,680 to Logothetis et al. It is termed a planar oxygen pumping sensor and includes first and second oxygen ion conductive solid electrolyte material layers, the first electrolyte material layer having greater porosity than the second electrolyte material layer, and first, second and third electrodes. The first electrode is between the first electrolyte material layer and the second electrolyte material layer. The second electrode is on the first electrolyte material layer. The third electrode is on the second electrolyte material layer. In this planar oxygen pumping sensor, only three electrodes are required to form an oxygen pump and an oxygen sensor. The more porous first electrolyte layer material acts to provide an enclosed volume with an aperture for establishing an oxygen gas reference partial pressure, similar to the structure of devices disclosed in the Hetrick et al. patents mentioned above.
As discussed above, oxygen pumping sensors possess several advantages over oxygen sensors such as the oxygen concentration cell. These advantages are higher sensitivity and less temperature dependence, and less dependence (even none) on the absolute gas pressure. Compared to the oxygen concentration cell, the oxygen pumping sensors have the additional advantage of higher signal levels (volts compared to millivolts for the oxygen concentration cell) and generally lower sensitivity to electrode properties. On the other hand, the oxygen pumping devices need calibration. If structural dimensions of oxygen pumping devices could be controlled and reproduced accurately, the need for calibrating individual devices would be minimized or even eliminated, but for one such "standard" device. It would also be desirable if the devices could be of a design which afforded a very low impedance for the device and a fast response time. Still further, it would be desirable if the fabrication techniques could be simplified so as to make the devices by a "batch" technique which would lead to lower costs. These are some of the advantages this invention offers.